Electrolytic treatment of chromium alloys



May 30, 1944. G. J. BEcKwlTH ELECTROLYTIC TREATMENT OF CHROMIUM ALLOYS Patented'May 30, 1944 ELEcTaoLmc TREATMENT oF CHROMIUM Glenwood J. Beckwith, Brecksville, h10, or to The American Steel and Wire of New Jersey,

ALLOYS assign- Company a corporation of New Jersey Application June 14, 1941, Serial No. 398,160

3 Claims.

The demand for highly lustrous iron alloys of the class known as stainless or rustless steels is increasing daily, owing to the increasingly widespread use of such materials as ornamental and sanitary finishes and ilttings in innumerable applications of stainless steel. However, in general, the production of the requisite lustrous linish to meet popular demand is attended with :onsiderable diiiculty.

At the` time when stainless steel was introduced generally on the market, a good metal surface, which has been whitened by pickling, was the only material that was available practicably. But as a use of such steels increased, there arose a demand for highly lustrous surfaces, which, because of their appearance, became utilized in both ornamental and utilitarian adaptations. of demanded finishes, the articles were subjected to mechanical grinding, polishing and burnish` As the demands for products of this character increased, however, such mechanical procedures became too slow to fuliill the requirements. Consequently, investigators turned to chemical and then to electrochemical means, for producing the desired surface iinish.

Chemical methods, while producing iinished surfaces satisfactory for many purposes, nevertheless proved to be equally inferior to the nishes produced by mechanical operations. Additionally, control of chemical procedures is difficult, owing to variations in composition and character of the metal being treated, inherent irregularities in the surface, and many other factors which render a chemical treatment inconsistent in its effects.

Electrochemical methods have been tried with success, the procedure being to make the article being polished the anode in an elctrolytic cell wherein the electrolyte may be any one 0i' a In order to effect the productionl bright polish on the round wires which make" up these articles.

As has been indicated above, steels of this character may contain substantial amounts of nickel in addition to chromium, or, the nickel may be omitted and the percentage of chromium increased to give the straight rustless chromium steels.

Since nickel is a. strategic metal with quantities thereof not sufllcient, generally speaking, to fulfill defense requirements in national emergencie's, under such. conditions attention becomes placed more concentratedly upon the straight chromium-iron alloys, which are thematerials being found customarily as round wires in refrigerator, and other, shelves, as well as various other applications wherein stainless steel variety of polishing solutions adapted for the purpose, or a mixture of various components intended to have a brightening action on the anode during electrolysis.

However, wide varieties of alloys are classed under the general heading of stainless or rustless" steel. Some of these contain substantial amounts of nickel, while others are substantially straight chromium-iron alloys.

Additionally, the adaptation of such compositions in the form=of round wires for the production of refrigerator shelves and other applications is becoming of increasing importance, with the attendant necessity of obtaining a uniform is employed.

In view of the diierence in composition, many procedures which are satisfactory in the case of the nickel containing stainless steels are not satisfactory for straight rustless chromium steels.

'I'he present invention provides an improved process for electropolishing straight chromium rustless steels in whatever form, being adapted equally to the polishing of ilat shapes as well as round wires or any other specific shape of metal.

In general, it has been found that stainless steels may be polished by making them an anode in a bath containing phosphoric acid, although other reagents may be utilized as an electrolyte. Thus, the electrolyte may be a highly concentrated solution of sulphuric acid, or chromicacid, or phosphoric acid, or mixtures of sulphuric and phosphoric acids, sulphuric acid and certain organic acid such as citric acid, or sulphuric and hydrouoric acids. Phosphoric acid electrolytes have certain advantages as to economy and ready control of reactive properties, the usual electrolyte being composed of a concentrated aqueous solution of orthophosphoric acid.

It is well known that electrolytes are more conductive at elevated temperatures than they are at room temperatures. and in order to avoid an undulated or pitted'surface, considerable care must be exercised in the matter of concentrations, current densities and other factors, particularly where stronger acids may be employed. Orthophosphoric acid appears to' give the best results with the least attention as to control of conditions in the electrolyte.

Also, it is well known that certain organic materials serve to inhibit pitting by polarization phenomena, as is true, for example, in the chemical pickling of steels, but it is found that such materials do not operate in this way in the presence of' an electrolyzing current with i the treated article as an anode in the bath.

Bath composition by weigh Pel' Gent Orthophosphoric acid (85% grade) 81.0 15 Sulphuric acid (60 Baum) 117.7 Alcohol l-- .37 Fluorescein .03

In preparing the bath, the alcohol and :iluorescein are mixed separately and a small amount of acid' is added to it. It is then mixed into the main electrolyte.

The optimum conditions of electrolysis using the foregoing bath composition are as follows:

26 Current density 4 amps. per square inch Voltage 5 to 6 volts Temperature 14o-160 F. Time l 5 to 7 minutes (depending on surface conditions) Results A very bright and lustrous finish especially on cold drawn straight chromium rustless steel round wire containing 17% chromium.

The above bath operates more smoothly without the sulphuric acid but the finish is not nearly so bright. A content of more than 20% of 60 Baum sulphuric acid severely pits both 17% and 12% straight rustless chromium steel round articles even at increased current density.

Low current densities accompanied by low voltages are conducive to iron solution in preference to chromium, and a medium current density, for example, 4 amps. per square inch passivates or polarizes the iron Yareas so that a uniform dissolution of the surface is realized.

The temperature is found to be best kept with'- in the limits specified above, since too high a temperature tends to pit the stock, and too low a temperature is not conducive to a brilliant luster. I That the presence of alcoholic fluorescein in the bath changes the polishing characteristics of the bath is discernibleby the fact that some foaming is noticed and a decidedly diifere'nt appearance of the polished surface results after adding the alcoholic fluorescein.

It is noticed in the practice that if straight viiuorescein is added to the phosphoric acid, the

luster of the stock is not greatly enhanced, :and if sulphuric acid is added to the foregoing bath, the luster is improved only slightly and the degree of pitting is greater. But, on the other hand, if the i'luorescein is dissolved in alcohol and then is added to the phosphoric acid as alcoholic .fiuorescein, the addition being made prior to the sulphuric acid addition, the stock is rendered to be about twice as lustrous as otherwise.

The accompanying drawing illustrates a diagrammatic sectional 'elevation of anapparatus for carrying out the present invention.

In the single view of the drawing, the article 2 to be polished is hung into the bath 4 on a hook or clamp 6 capable of carrying the current required. The bath 4 is contained in a suitable $01!- tainer I, which may be composed of acidproof brick, or other suitable material.

The article 2 forms an anode and its suspending hanger 0 engages a copper bus-bar il'which is connected by current lead il to the positive terminal i2 of a suitable current generator i6.

Cathodes I8 of suitable material are spaced properly around the article 2, these cathodes being connected through bus-bars i8 and leads 2l, 22, 23 to the negative terminal 2t of the generator it, which for convenience is shown as being mounted on a lshelf projecting from, the cell.

The bus-bars form permanent connections and are conveniently located above the level of the electrolyte to avoid excessive action of electrolyte spray thereon.

In operating, it is highly important that a unlform flow of current be obtained from all parts of the surface, in order to avoid shaded or undulated areas.

Also, the hanger 0 is designed to assure an even distribution of current over the entire area of the anode 2 being polished. The hanger 6 is insulated in all'parts exposed to the bath except on the area making intimate contact with the article being polished in order to prevent dissolution of the hanger and wastage of current.

1t will be understood, of course, that the invention is not limited to the use of a phosphoric acid bath, or a mixture of phosphoric acid and sulphuric acid, containing alcoholic fluorescein as has been preferentially expressed herein, it having been found that, generally speaking, the pres- `ence of alcoholic fiuorescein in any of the electrolytes used customarily for eiectropolishing stainless steels has a benencial effect.

It will be understood that the foregoing composition of the bath as stated above, is the preferred composition which produces best results. However, the specific composition may range, in the completed electrolyte, from to 90% of orthophosphoric acid grade), from 10% to 20% of sulfuric acid of 60 Baume grade, and appreciable amounts of alcoholic iluorescein, but iessthan 1% thereof.

It is found in practice that, however, in the case of straight chromium rustless steels, the preferred electrolyte avoids a appearance of blueness or fogginess on the ace, but produces a mirrorlike uniform finish at low cost both for material and current consumption.

It will be understood that the indicated ,percentages of components of the bath as given in the claims are percentages by weight in each instance.

I claim: Y

l. The process of producing highly polished surfaces on corrosion-resistant chromium steels, which consists in making the steel an anode in a bath containing approximately 81.9% by weight of orthophosphoric acid of 85% grade, 17.7% of sulphuric acid of 60 Baume gravity, .37%-by` weight of ethyl alcohol and .03% of iluorescein, the balance being essentially water, the fiuorescein havingbeen added to the bath as a solution thereof in the alcohol prior to addition of sulphuric acid to the bath, providing suitable cathodes to assure uniform current flow from all parts of the steel anode, maintaining the electrolyte at temperatures between about to about 160"..F. and passing a current through the'resulting cell at a current density of substantially 4 amps. per

square inch of anode surface and a voltage of from about 5 to about volts. v

2. The process for producing highly polished surfaces on corrosion-resistant chromium steels,

whichconsistsinmaglngthesteelananodeinan aqueous electrolyteoonsisting of from 80% to 90% by weight of orthophosphoric acid of 85% grade, from approximately 10% to approximately 20% by weight of sulphuric acid of 60' Baume, .37% by weight of ethyl alcohol and .03% by weight 'of iiuorescein, the balance being essentially water, the iiuorescein having been added to the bath as a solution thereof in the alcohol and prior to addiallpartsofthesteelanode,andpassinacur rent through the resulting cell of ysuiiicient density end for e eumexent length ef time to effect al polish on the said metal surface.

3. An electrolyte :or use in the medie polishing of stainless steel, consisting of an aqueous solution containing in the ilnished solution, between 80 and 90% by weight of orthophosphoric acid of 85% grade, between 10% and 20% byweight of sulphuric acid of 60 Baum, and .03% byl weight of fiuoresceiri' dissolved in ethyl -alcohol amounting to .37% by weight of the electrolyte, the balance of the electrolyte being water'.

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